The invention relates to a method of manufacturing a body moulded from a plastic material covered with a metallic layer, which is applied in the manufacture of flat hyperfrequency aerials composed of radiating elements with microstrip lines associated with a dielectric substrate.
Such aerials are described in French patent applications Ser. Nos. 82 18 700 and 83 06 650 filed by Applicants on Nov. 8, 1982 and Apr. 22, 1983, respectively.
For receiving or transmitting a signal having a single polarization, such aerials comprise two metal plates in which are provided on the one hand apertures directed towards the propagation medium, constituting the radiation elements, and on the other hand a pattern of grooves destined to receive central conductors. The control conductors, supported by a dielectric film enclosed between the two metal plates, constitute, with the radiation elements and the grooves, the supply network of the aerial. The total surface area of such aerials may be from a few square centimeters to several square meters.
Such aerials are of increasing interest due to the fact that they find their application in the reception of television emissions relayed by satellite. Various other structures of flat aerials have been proposed in the last few years, but they do not fulfil the conditions imposed by the specifications of the C.C.I.R. (Comite Consultatif International de Radio), while the flat aerials manufactured according to the above patent applications have low losses, a high gain and a wide pass-band in the wavelength ranges in question.
In these conditions it is imperative to provide a manufacturing method permitting the manufacture of such aerials in large quantities at low cost. In fact, the applications in the field of television reception always imply large scale manufacture for a range of products showing a satisfactory ratio between the quality and the price.
Therefore it is advantageous in the manufacture of such aerials to replace the metal plates by metallized plastic plates, at the same time giving the assembly a great resistance to wear and weather conditions.
To effect this the present invention relates more in particular to a method of manufacturing a body moulded from a plastic material and coated with a metallic layer, according to which an intermediate layer for facilitating removal from the mould has previously been formed at the surface of the mould; the metallic layer is deposited in the mould before the plastic material is introduced.
It is known from British Pat. No. 1167690 published on Oct. 22, 1969, to manufacture such bodies from metallized plastic. The method suggested by the British Pat. specification includes covering a wooden mould with a first layer of polyvinyl alcohol which is destined to promote the removal from the mould and which will be eliminated during operation. A metallic layer is then pulverized on the polyvinyl alcohol in the mould thus prepared and the plastic body is injection-moulded. It then suffices to dissolve the polyvinyl alcohol layer to obtain the plastic body covered with the metallic layer.
Such a method has the disadvantage, already previously mentioned in this document, that only a small number of bodies can be manufactured. Moreover, the polyvinyl alcohol layer provided to avoid the adherence of the metal layer to the mould, does not enable molding, with high precision dimensions, the details of the moulded body. In effect it does not have a uniform thickness over a large surface. Also the method described in the British specification does not enable the manufacture of bodies of large dimensions.